Understanding Decision Structures in Java: If-Else and Logical Operators

Decision Structures: The If-Else If Statement, Nested If Statements, Logical Operators, and String Comparison CS0007: Introduction to Computer Programming

Review • In a decision structure’s simplest form certain statements are executed only when… • a specific condition exists. • It is said that the statements inside of the decision structure are… • conditionally executed. • Relational Operators determine whether… • a specific relationship exist between two values. • Some relational operators in Java are… • >,< , >= , <= , == , !=

Review • General for of an if statement: if(BooleanExpression) statement; if(BooleanExpression) { statement1; statement2; ... } • A Flagis… • a boolean variable that signals when some condition exists in a program. • When Java compares characters, it compares the character’s… • Unicode values

Review • General form of an if-else statement: if(BooleanExpression) statement or block 1 else statement or block 2

The if-else if Statement • Sometimes you need to be able to test a series of conditions • You can do this with the if-else if statement • General form: if (BooleanExpression1) statement or block 1 if else (BooleanExpression2) statement or block 2 else statement or block 3 • If BooleanExpression1 is true, then statement or block 1 is executed. • If BooleanExpression1 is false, then BooleanExpression2 is tested. • If BooleanExpression2 is true, then statement or block 2 is executed. • If BooleanExpression2 is false, then statement or block 3 is executed. • Note: You can have as many if elseclauses as is needed.

if-else if Example • New Topics: • if-else ifStatement • Using else as an error case

Nested if Statements • Nesting is enclosing one structure inside of another. • A block in Java can contain any valid Java code, this includes other if statements: if(BooleanExpression1) { if(BooleanExpression2) { statement1; statement2; } statement3; statement4; } • If BooleanExpression1 is true and BooleanExpression2 is true, what is executed? • statement1 ,statement2 , statement3 , statement4 • If BooleanExpression1 is true and BooleanExpression2 is false, what is executed? • statement3,statement4 • If BooleanExpression1 is false, what is executed? • Nothing

Nested ifStatements Example • New Topics: • Nested if Statements

Logical Operators • Java provides logical operators. • The binary logical operators combine two boolean expressions into one. • The unary logical operator switches the value of a boolean expression. • Binary logical operators have lower precedence than relational operators (they will be evaluated after) • NOT has the same precedence as negation.

Logical Operator Truth Tables • Truth Tables show the result of a logical expression based on the values of the operands.

Logical Operator Practice • 2 > 3 && 4 < 5 • false – first operand is false • 2 < 3 && 4 < 5 • true • 2 > 3 || 4 < 5 • true • 2 > 3 || 4 > 5 • false – both operands are false • !(2 > 3) • true– operand is false

Logical AND Example • New Topics: • Logical AND

Logical AND and Nesting • If we have the && operator, do we need nesting? if(BooleanExpression1) { if(BooleanExpression2) { Both conditions met } else{ Both conditions NOT met } } else { Both conditions NOT met } if(BooleanExpression1 &&BooleanExpression2) { Both conditions met } else { Both conditions NOT met }

Logical AND and Nesting • Answer: Yes, to act if one of the conditions is not met: if(BooleanExpression1) { if(BooleanExpression2) { Both conditions met } else { Condition 2 not met } } else { Condition 1 not met } if(BooleanExpression1 && BooleanExpression2) { Both conditions met } else { Both conditions NOT met }

Exercise • Change NestedIfStatement.java to tell the user if she does not meet one requirement, the other requirement, or BOTH. • Hint: we talked about something last lecture that will help us… • Flags

Logical OR Example • New Topics: • Logical OR

Exercise • Change LogicalOrOperator.java to allow the user to enter capital or lower case Y or N as an answer. • Hint • Use another OR operator. • Also, check to make sure the user entered a valid response. • Y, N, y, or n.

Comparing String Objects • Imagine you have declared two String variables as such: String x = "buffalo"; String y = “bison"; • What does x == y resolve to? • false, but not for the reason you think! • The == operator compares what the reference values are (what objects they are pointing to), NOT what the value of the string is. • In some cases this causes problems • You should use methods in the String class in order to compare String variables • equals • compareTo

String equals method • To check if two strings are the same you can use the equals method. StringReference.equals(OtherString); • If the string referred to by StringReference is equal to the one referred to by OtherString, then true is returned. • Otherwise false is returned. • This comparison is case-sensitive ("buffalo" and "Buffalo" are not equal) • To do case-insensitive comparison, use equalsIgnoreCase

String compareTomethod • If you want to determine which string is “greater” use the compareTomethod. StringReference.compareTo(OtherString); • If the string referred to by StringReferenceis equal to the one referred to by OtherString, then 0is returned. • If the string referred to by StringReferenceis “less than” to the one referred to by OtherString, then a negative number is is returned. • If the string referred to by StringReferenceis “greater than” to the one referred to by OtherString , then a positive number is returned. • This comparison is case-sensitive ("buffalo" and "Buffalo" are not equal) • To do case-insensitive comparison, use compareToIgnoreCase

String compareTo method • What does it mean for strings to be “greater than” or “less than” each other? • Java compares the two strings, character by character from left to right. • When there is a difference in a character it compares the Unicode values of the characters. • If a string is shorter than another, and the shorter one is the beginning of the longer one, the shorter one is considered less. • "hi"is less than "high"

String Comparison Example • New Topics: • String==operator • equals method • compareTo method

Block-Level Scope • If a variable is declared inside of a block, it is said to have block-level scope. • If a variable has Block-Level Scope it is in scope from its declaration to the ending of the block in which it was declared. if(BooleanExpression) { intx; … } • The variable x has scope from the point it was declared to the } • This does not have much use now, but will be more useful when we learn loops.

The Conditional Operator • Java provides and operator to create short expressions that work like if-else statements. BooleanExpression ? Value1 : Value2; • If BooleanExpression is true, Value1 is returned • If BooleanExpressionis false, Value2is returned • Example: if(score < 60) System.out.println("You Fail"); else System.out.println("You Pass"); System.out.println("You " + score < 60 ? "Fail" : "Pass");

Understanding Decision Structures in Java: If-Else and Logical Operators